1. Technical Field
The present invention relates to a motor control device, a robot hand, a robot, and a motor control method.
2. Related Art
A method of controlling a stop position of a movable part using a servomotor in a robot or a machine tool has been widely used. When a robot hand grasps a grasped object or inserts a member into a hole, the movable part may not move before the movable part reaches a prearranged position. At this time, a system for setting a torque limit in a motor and controlling a movable part is disclosed in JP-A-5-6210. According to this technique, the control system includes a storage unit storing plural torque limit values and switches the torque limit values for every work detail carried out by a motor or a movable part to be driven.
A method of improving the controllability of a rotational speed without rapidly changing a torque during the rotational driving of a motor is disclosed in JP-A-2011-147268. According to this technique, in a speed control of the motor, a deviation between an instructed speed value and a detected speed value is amplified by the use of a speed regulator including a proportion-integration adjuster to calculate an instructed torque value. A method of controlling the current of the motor so as to match the actual torque with the instructed torque value is employed. Here, the optimal adjustment of the speed regulator is performed on the basis of the value of the moment of inertia. A torque feed-forward compensator calculates a torque feed-forward compensator using the value of the moment of inertia and the instructed speed value. An adder adds the output of the speed regulator and the torque feed-forward compensation value to calculate the instructed torque value. When a robot hand grasps a grasped object, first, a control device causes a movable part of a robot hand to approach and then contact the grasped object. Subsequently, a state where the movable part presses the grasped object is maintained. When causing the movable part to approach the grasped object, the control device controls the speed of the motor. After the movable part contacts the grasped object, the control device switches the control of the motor to a torque control. The torque of the motor is controlled so that the movable part can press the grasped object with a force with which the grasped object can be grasped. At this time, the control device can cause the movable part to grasp the grasped object by freely controlling the torque of the motor depending on the situation while controlling the motor.
When the control of the motor is switched from the speed control to the torque control, a method of switching a control circuit performing the speed control to a control circuit performing the torque control has been used up to now. Here, in order to switch the control system, it is necessary to temporarily stop the movement of the movable part. Accordingly, there is a need for a method of causing the robot hand to grasp a grasped object without temporarily stopping. That is, there is a need for a control device of a motor of which the rotating shaft rotates with a torque not greater than a set torque limit value while controlling the speed even when the rotating shaft is rotating and when the rotating shaft is stopped.